In image forming apparatus (electrophotographic copying machine, electrostatic recording apparatus, etc.) such as copying machines according to an electrophotographic system, printers and facsimiles, images are formed through steps such as charging, exposure, development, transfer and fixing. In such an image forming apparatus, a toner image formed on a photosensitive drum through the steps of charging, exposure and development is transferred to a transfer material such as transfer paper or OHP film and then fixed to the transfer material by a means such as heating. In the transfer step of the image forming apparatus, there is used a transfer material-carrying member having the functions of carrying a transfer material, conveying the transfer material to a transfer position and separating the transfer material after transferring a toner image to the transfer material to deliver it to a fixing step.
A dielectric or conductive film has heretofore been used as such a transfer material-carrying member. In many cases, the transfer material-carrying member is formed in the form of a drum or endless belt. It is very important that a material used for the transfer material-carrying member should be a material capable of controlling the electrical characteristics of the transfer material-carrying member within preferred ranges. At the same time, it is also necessary for such a material to have excellent mechanical characteristics, because the transfer material-carrying member is required to play a role as a mechanical structure.
A resin material comprising a polycarbonate resin as a main component has been proposed as a resin material for transfer material-carrying members (Japanese Patent Application Laid-Open No. 311472/1995). In this technical field, besides this material, polyester resins such as polyethylene terephthalate and polybutylene terephthalate, fluorocarbon resins such as polyvinylidene fluoride, and the like are used as resin materials. However, these resin materials have both merits and demerits and do not fully satisfy high electrical characteristics and mechanical physical properties required of the transfer material-carrying members.
For example, a transfer material-carrying member formed from a polycarbonate resin exhibits far excellent fatigue endurance, but is not sufficient in electrical characteristics such as dielectric constant and also insufficient in tensile elongation at break. More specifically, the polycarbonate resin is excellent in stiffness or rigidity, but, on the other hand, it has high brittleness. Therefore, the transfer material-carrying member formed from the polycarbonate resin tends to cause brittle fracture when it passes the mechanical endurance limit. This suggests the possibility that the polycarbonate resin may be lacking in the reliability as a mechanical part. In order to provide a transfer material-carrying member as a high-performance member, it is desirable that the member should have good stiffness and cause ductile fracture. However, the resin material comprising the polycarbonate resin as a main component is not sufficient in these characteristics.
In addition, the polycarbonate resin has a low dielectric constant. Polyester resins also have not a very high dielectric constant. Charging and discharging are repeated on a transfer material-carrying member upon use. If the dielectric constant of the transfer material-carrying member is low, it is necessary to make applied voltage higher. More specifically, a charge level upon charging operation is represented by the equation (1): EQU Q=CV (1)
wherein Q is charge, C is capacitance, and V is applied voltage. If C (.alpha. .epsilon.: dielectric constant) is small, it is necessary to make V higher in order to obtain a fixed Q. In order to make V higher, it is necessary to make an electric power unit larger. Therefore, the apparatus cost is increased as a whole. When high voltage is applied, discharge toward peripheral metal members and the like tends to occur, and so the necessity of taking insulation measure arises.